Number of millennials agreed upon, give or take 66M

The generation that came of age around the turn of the millennium gets a lot of criticism from the left and the right (and anonymous Internet commenters) for everything from being lazy and indolent to not being able to drive a stick shift.

As CNBC recently reported, the definition of a millennial depends greatly on who's doing the defining. Studies, surveys and newspaper articles reviewed by CNBC ranged in their definitions of "millennials" from those born in 1976 to 2010, a 36-year span than includes both the height of disco and the Obama administration.

This is important because it's the hope of reaching "millennials" that is often sold by marketing agencies to their clients hoping to reach this key demographic. While younger consumers tend to have less disposable income, they're often more frivolous with how they spend the money they do have, and reaching consumers when they're young can help companies capture life-long customers.

So when companies buy advertising directed at "millennials," they're targeting somewhere between 39 million Americans and 105 million, depending on who's doing the selling.

FutureCast, a marketing consultancy, puts millennials' birth years between 1977 and 2000, meaning 95 million, according to birth data from the CDC. That's 30 percent of the American population and nearly 40 percent of adults.

At the same time, this group makes between 21 and 25 percent of consumer discretionary purchases, according to Jeff Fromm, FutureCast president and author of a number of books on how to market to millennials.

So-called millennials may not have the purchasing power of other generational groups, but they have outsized power in the marketplace by influencing the purchases of older folks.

"I'm 49 years old," Fromm said. "If I was going to buy new tech, I'd call my daughter" for recommendations.

More important than strict age groups, according to Fromm, is the "millennial mindset," which is more a product of hyper connection. "We now have a lot of older consumers using content to make their decisions, using technology to change how they operate day to day," Fromm said.